1. Field of the Invention
This invention relates to a apparatus and a method for pulling up single crystals, which apparatus is suitable for installing on an semiconductor single-crystal manufacturing equipment utilizing the CZ method, especially suitable for the process of pulling up heavy single crystals.
2. Description of the Related Art
Conventionally, single crystal silicon is produced by the CZ method. In the CZ method, polycrystalline silicon is deposited into a quartz crucible of a single-crystal manufacturing equipment, then the polycrystalline silicon is heated to melt by way of heaters disposed around the quartz crucible, subsequently a seed crystal installed within a seed holder is dipped into the melted liquid. Subsequently, the seed holder and the quartz crucible are respectively driven to rotate in the same or reversed directions. At the same time, the seed holder is lifted to grow a single crystal with predetermined diameter and length.
In the seed crystal, dislocation will arise due to the heat shock occurring at the time of dipping it into melted liquid. In order to stop propagation of dislocation from the seed crystal to single crystal to be grown, a necked portion with diameter from 1 to 4 mm is grown up beneath the seed crystal so as to release the dislocation to the surface of the necked portion. Making certain that the grown crystal grow dislocation-free state, a shoulder is formed and the single crystal is enlarged to comply with a predetermined diameter. Then, the operation of forming the body of the single crystal is performed.
In recent years, following the enlargement of single crystals, the weight of an individual single crystal is increasing, and the strength extent of the necked portion is reaching its limit. To solve the problem, a necked portion with a diameter larger than that of the necked portion is formed on the top of the shoulder when the propagation of dislocation has been stopped by way of the reduction process. It is possible to hold the single crystal by the necked portion so as to prevent breaking of the necked portion. Alternatively, measures are taken to prevent dropping of the single crystal even if the necked portion breaks. For example, in the single-crystal pulling up apparatus disclosed in Japanese Patent Publication Gazette TOKU KOU HEI 5-65477, a clamp arm capable of being driven to open or close is engaged with the necked portion of a single crystal so as to suspend the single crystal. Furthermore, in the single-crystal pulling up apparatus disclosed in Japanese Patent Publication Gazette TOKU KOU HEI 7-515, a plurality of claws retained at a predetermined angle are provided at the lower end of an engaging holder capable of being driven to ascend or descend. The claws are engaged with the necked portion of a single crystal so as to suspend the single crystal In addition, in the single-crystal pulling up apparatus disclosed in Japanese Patent Publication Gazette TOKU KOU HEI 7-103000, a plurality of claws engaging with the necked portion of a single crystal are provided. The single crystal is suspended by utilizing a plurality of holding levers capable of being driven to open or to close by their ascending or descending movement and a ring used for preventing opening of the holding levers.
However, in the single-crystal pulling up apparatus disclosed in Japanese Patent Publication Gazette TOKU KOU HEI 5-65477, the clamp arm will be driven to open when the weight of the single crystal is getting increased, and there exists a danger of dropping of the single crystal during pulling up operation. However, in the single-crystal pulling up apparatus disclosed in Japanese Patent Publication Gazette TOKU KOU HEI 7-515 and TOKU KOU HEI 7-103000, there exists a danger that the single crystal being pulled up may convert into a polycrystal. That is due to the mechanic shock or vibration induced in the process of clamping the single crystal. In addition, the single crystal and the claws are kept in contact at several points. Therefore there exists a danger that the single crystal may be wrecked. Besides, it is impossible to take the single crystal off from the clamping means, once the single crystal has been clamped and installed in the main chamber. Thus, even if polycrystallization of the single crystal being pulled up has arisen, it is impossible to remelt the single crystal.